Direct current to alternating current converter



Nov. 10, 1964 s. L. MERKEL 3,156,377

DIRECT CURRENT TO ALTERNATING CURRENT CONVERTER Original Filed April 25.1958 2 Sheets-Sheet 1 INVENTOR.

STEPHEN L. MERKEL M WM Nov. 10, 1964 s. L. MERKEL 3,156,877

DIRECT CURRENT T0 ALTERNATING CURRENT CONVERTER Original Filed April 25,1958 2 Sheets-Sheet 2 INVENTOR. ST EPHEN L. MERKEL zM/M United StatesPatent F 3,156,377 DIRECT (IURRENT T0 ALTERNATING CURRENT (IGNVERTERStephen L. Merirel, Cleveland, Ohio, assignor to Lorain ProductsCorporation, a corporation of Ohio @riginal application Apr. 25, 1958,Ser. No. 730,962, now Patent No. 3,065,429, dated Nov. 2%, 1962. Dividedand this application Aug. 28, 1962, Ser. No. 219,897

4 Claims. (Cl. 331-113) This application is a division of applicationSerial No. 730,962, filed April 25, 1958, now US Patent No. 3,065,429,granted November 20, 1962.

This invention deals with direct current to alternating currentconverters, and particularly with a transistorized converter suitablefor use as a ringing current supply in a telephone central ofiice. Thealternating current used for operating the ringers in telephonesubscribers sets has in the past been supplied from one of severaldifferent types of apparatus, including rotary generators, driven byeither an alternating current or direct current motor, alternatingcurrent operated frequency converters, and vibratory pole changers.Ideally, the telephone ringing current generator must be reliable, andcapable of operating continuously over a period of many years withoutfailure and with a minimum of attention. The load which the source ofringing current must supply varies widely, and under abnormal operatingconditions may greatly exceed the rated capacity of the source ofringing current. Therefore, the ringing current supply must be capableof operation under heavy overloads without damage to the equipment, andmust resume normal operation as soon as the overload condition isremoved.

In many telephone systems, selective signalling is achieved withmechanical resonant ringers operated selectively from a multi-frequencyringing current source. For this service, the frequency stability of theringing current source is of the utmost importance, and the harmoniccontent or" the ringing voltage is also important. In somemulti-frequency systems, the ringing frequencies are in harmonicrelationship to each other, so that a harmonic of a lower frequency cancause a false signal by operating a ringer of a higher frequency. Asinusoidal wave shape is, therefore, preferred for ringing voltage, asit eliminates the danger of cross-ring due to harmonics. At the sametime, the presence of frequencies in the audio range of above,approximately 700 cycles is undesirable, because these frequencies whenapplied at the levels asso ciated with ringing current can readily crossover into adjacent circuits and cause undesirable cross-talk.

Although the higher audio frequencies mentioned above are undesirable,the presence of an audible component in the ringing voltage is requiredin many telephone systems to signal the calling subscriber when thecalled subscribers bell is rung. The ideal ringing current generator,therefore, includes an audio frequency component in the range around 500cycles, and of a controllable magnitude.

Because telephone ringing current must be supplied even in the absenceof the normal alternating current power, a direct current operatedringing current generator is preferred, but a direct current operatedgenerator must have the highest possible efiiciency, because anyincrease on the direct current consumption in the telephone ofiicerequires the installation of larger batteries, larger chargingequipment, and results in increased alternating power consumption.

The telephone ringing current generators of the prior art have allfallen short of the ideal on one or more of the requirements outlinedabove. The direct current to alternating current converter of thepresent invention 3,156,377 Patented Nov. 10, 1964 ice overcomes themajor shortcomings of prior devices, and approaches the idealcharacteristics as will be more fully explained in the followingdisclosure.

It is an object of my invention to provide a source of telephone ringingcurrent operating from a direct current supply and employing transistorsfor converting the direct current to alternating current.

Another object of my invention is to provide a direct current toalternating current converter which has a relatively constant outputvoltage in spite of normal load current variation and in spite ofvariations in the direct current source voltage.

A further object of my invention is to provide a transistorized directcurrent to alternating current converter which will maintain relativelyconstant voltage for normal loads and which will not be subject todamage in case of overload or short circuit.

An additional object of my invention is to provide a transistorizeddirect current to alternating current converter having an output voltageapproaching a sinusoidal wave shape.

A further object of my invention is to introduce into output voltage ofmy converter an audible tone signal of controilable frequency andamplitude.

A further object of my invention is to maintain a stable outputfrequency in a direct current to alternating current converter in spiteof varying load, varying direct current source voltage, variations ofambient temperature, and prolonged continuous use.

An additional object of my invention is to supply an alternating currentload from a direct current source with maximum efficiency, and tomaintain high efiiciency throughout a wide range of load variations.

A still further object of my invention is to provide means for adjustingthe output voltage in a direct current to alternating current converter.

An additional object of my invention is to provide in a trausistorizeddirect current to alternating current converter, a power amplifierwhich, when energized from a substantially constant voltage source, ischaracterized by substantially constant output voltage for normal loadcurrents and greatly reduced output voltage for excessive load currents.

Other objects and a fuller understanding of my invention will beobtained from the following specification and claims taken inconjunction with the accompanying drawings in which:

FIGURE 1 is a schematic diagram of a preferred embodiment of myinvention comprising a voltage regulator circuit supplying power to anoscillator and an intermediate amplifier, and comprising a poweramplifier for supplying power to a variable load, and

FIGURE 2 is the schematic diagram of a modified power amplifier whichmay be applied to the circuit of FIGURE 1.

With particular reference to FIGURE '1, there is shown a voltageregulator designated by the general reference character supplying powerto an oscillator circuit designated by the general reference character61 and to an intermediate amplifier designated by the general referencecharacter 62. The output of the oscillator 61 is fed through theamplifier 62 to a power amplifier designated by the general referencecharacter 63 which supplies power to the load 16. A tone generatingcircuit designated by the general reference character 67 is a portion ofthe oscillator circuit 61. The tone generating circuit is provided withterminals 38a and 3911 which are connected to the upper end of Winding65 and the upper side of capacitor 25, respectively.

The voltage regulating circuit 60 makes use of a breakdown diode, orZener diode 38 having a substantially constant voltagecharacteristic.The diode 38 is connected in series with resistor 39"across the voltageto be regulated. A voltage divider comprising resistors 68, 24 and 69 isalso connected across the voltage to be regulated. AtransistorIZ3'of1PNPtype is shown with its base connectedito theisliderof'adjustable resistor 24 and its emitter connected to the junctionbetween diode 38 and resistor 39; The-substantially constant voltageacross diode 38 is thus compared with' a portion of the voltage to beregulated appearing across resistor 68' and a portion of resistor 24. Incase the voltage across-this portion of the voltage dividing circuit isless than the voltage across diode 38,:the bas'ezof transistorZS will benegative withrespect toits emitter, 'andttransistor 23 -will conduct.The conductionz of transistor23 supplies current to the base oftransistor 22, causing this transistor to conduct, and raisingthe-voltageuntil 'the'voltage at the base of transistor 23 approximatesthe voltage across'the' diode 38. The voltage regulating-circuitdescribed is of a type known in the art, anduis shown only by wayof'example, as other devices known to those skilled in the art may beused for regulating: thesvoltage to the oscillator and amplifier of myinvention; 7

The regulated'voltage-is applied to two substantially equal resistors74) and 71 to provide a mid-tap to the regulated voltage supply.Capacitor 73 is connected across resistor 71' and capacitor 72'isconnected across resistor 70 to provide a path' for alternating currentimpressed'on the mid-tapof the direct current source.

Theoscillator 61 shown in FIGURE 1 is of a multivibrator type,comprising transistors 2'7 and 28, having their collectors energizedthrough resistors 74 and 81' respectively from the negative output ofvoltage regulator 6t); The base of transistor 28 is coupled to thecollector of. transistor .27 through capacitor 76. The base oftransistor 27" is coupled to the collector of transistor 28 throughcapacitor 77. A resonant circuit which establishes the frequency'ofoscillation is connected between the base of transistor 27' and;the baseof transistor 28. The resonant circuit comprises primary winding 65 oftransformer 26 which acts as an'inductance element, and capacitor 25.

whichisconnected inparallel with winding 65. Resistors 75 and'64intheemitter circuits of transistors 27 and'ZS," respectively, serveto'limit the emitter currentsof these:

transistors? Resistors 79 and Sit comprise a'voltage divider, theintermediate point of which is connected through resistors 78 and 66,respectively, to the bases of transistors 27' and 23 tc-providesufiicient bias-to insure starting of theoscillations. Resistors 7S ande providethe direct current potential'to-the bases of the transistors;these resistors must be of relatively high resistance because they areconnected directly across the resonant circuit which sets the frequencyof oscillation.

. The action of a multi-vibrator circuit such asthat shown is -wellknown and need not be described in detail here. The frequency ofoscillation is established by the resonant frequency of "capacitor 25-with the linear inductance element comprising transformer winding 65.is provided with taps for adjusting the frequency of oscillation. 1Other methods known in the art for adjusting the inductance orcapacitance in the circuit, such as a movable core in transformer-26mayalso be employed in adjusting the frequency.

The-tone generating;circuit-67 is connected in series with capacitor 25in the resonant'circuit. In this position it might be considered'aportion of the resonant circuit and, therefore, a major factor indetermining. the frequency of oscillation of the oscillator. However,one of the features of my invention is a tone generating circuit whichdoes-not interfere'with the normal operation of the oscillator.

The tonegenerating circuit 67 comprises a saturable Winding 65 Duringthe half-cycle of alternating currenfithroug capacitor 2 5 131 whichrectifier 31' conducts, the tone circuit is effectively shorted out.During the opposite half cycle, the current of capacitor 25 is forced tohow through thewinding of saturable inductance 29. Theinductance isthereby saturated, and a momentary voltage surge occursduringone portionof. the cycle. This voltage surge is impressed directlyon the WindingdSof trans former 26 and is thus transmitted to the intermediateamplifier. The magnitude of the voltage surge can be controlledby choiceof the taps on the tapped winding of saturableinductance 29.This'voltage surge represents an audio frequency signal of controllablestrength which is thus introduced into the output of the oscillator.

I prefer to connect a capacitor, such as the capacitor 39, in parallelwiththe saturable inductance element Capacitor 30 together withinductance 29 produces a damped oscillation in the-tonegeneratingcircuit and controls the frequency range of the audible-signalwhich is introduced into the output voltage of the oscillator.

With the tone circuit 67' as shown, I am able to'introduce an audiofrequency component of controllable fre quency and amplitude into thelow frequency output voltage of oscillator 61 while 'rnaintainingthebasic sinusoidalwave shape of this oscillator and without substantiallyaffecting the frequency of. oscillation.

Since the oscillator is supplied with regulated voltage from regulator64 the amplitude of the signal produced across transformer 25 issubstantially constant under all normal operating conditions.

An intermediate'or driver amplifier -62 is connected betweentheoscillator 61 and the power amplifier 63 in the preferred embodimentof my invention shown in FIGURE'I. The intermediate amplifier serves asa buffer between the power amplifier and the oscillator to preventchanges of load on the power amplifier from noticeably aife'cting thefrequency of oscillation. With sufficient gain in thepower amplifier,and with adequate power in the oscillator circuit, the intermediateamplifier may be omitted;

The intermediate amplifier 62' shown in FIGURE 1 comprises transistorsSZand 33'having their base-collector circuits energized from the secondarywindings 34 and 35 of'transforrner 26; Thecircuit shown is of thecornmon collector type designed for current amplification to Work intothe low impedance injut circuit of the power amplifier. amplifier:includes voltage dividers'cornprising resistors 83 and 36 connectedacross winding 34 and resistors 82 and 37 connected across winding 35.Resistors 36 and 37 are potentiometers having their sliders connected tothe bases of transistors 32 and 33 respectively. These twopotentiorneters are preferably ganged to provide a convenientadjustmentofthe output voltage. of the converter and, being ganged, inthe usual manner are movable in unison to- Ward or'away from one anotheras viewed in FIGURE 1. The transistors 321and 33 are operatedfrom thereduced voltageobtained from the mid-tap of voltage regulator 60; Theemitter-collector circuit of transistor 32 is thusenergizedisubstantially in parallel with capacitor 72 and theemitter-collector circuit of transistor 33 is energized substantially inparallel with capacitor 73. Coupling transformer 13has its primarywinding 45 enerized from the emitter-collector circuit of transistor 32,and its primary Winding 46 energized from the emitter-collector circuitof transistor 33.

The poweramplifier 63 m FIGURE 1 comprises transistors-,ll and 12 whichare energized with substantially The energizing circuit of theintermediate constant signal voltage from transformer 13. Because atransistor is inherently a constant current device, deliver ing anoutput current dependent upon the driving current, the provision of asubstantially constant voltage drive does not directly provide aconstant voltage output. In the practice of my invention, I employinverse voltage feedback and positive current feedback in the poweramplifier to obtain the desired results. The effect of inverse voltagefeedback is to stabilize the output voltage and to hold it substantiallyconstant even though the voltage of the direct current source it mayfluctuate over a considerable range. However, as the load 16 changes,the emitter-base current of the transistors also changes, re quiringmore driving current to provide increased load current. In addition, theresistances in the circuit elements, including the base resistances ofthe transistors, tend to cause a reduction in the output voltage as theload current increases. 1 am able to counteract this reduction involtage by the use of positive current feedback in the power amplifier,so that I can maintain a relatively constant load voltage for all normalvalues of load current. r

The emitter-base circuit of transistor 12 is energized from winding 47of transformer 13 through resistor 57, winding 5t of transformer lid andwinding 54- of transformer 15. Similarly, the emitter base circuit oftransistor 11 is energized from winding 43 of transformer 13 throughresistor 53, winding 51 of transformer 1d and winding 52 of transformer15. Resistors 57 and 5'8 are connected in series with the drivingcircuits to limit the base currents of transistors 11 and 32. Theseresistors may be omitted by designing the transformers with suitableinternal resistance their windings.

Primary winding 553 of output transformer 15 is energized from theemitter-collector circuits of both transistors by means of a centertapping arrangement applied to the direct current source If The centertapping arrangement comprises resistors 19 and 2%? with capacitors 1'7and 1S, respectivel providing alternating current paths to the centertap 8d. Transistors H and 12 operate a class B push-pull amplifier, inwhich each transistor conducts current during approximately one-half ofthe driving voltage cycle. While transistor 12 is conducting, a currentpath is provided through winding 53 from right to left, energizedsubstantially in parallel with capacitor 1'7. While transistor 11 isconducting a current path is provided from left to right through winding53, energized substantially in parallel with capacitor 18. The c1rcu1tis designed so that the maximum voltage each transistor must control isonly slightly above the voltage of source ltl. This enables the use ofcurrently available transistors in a highly eiiicient circuit operatingdirectly from the 50 volt battery normally available in telephonecentral ofiices.

Load current is supplied from secondary winding 5'6 of outputtransformer 15. The resistor 21 is connected between the load 16 and theoutput winding 5, to act as an impedance element in the current feedbackcircuit. The voltage developed across resistor Zl is applied to winding49 of transformer 14. Windings 5d and El of transformer 14 are connectedrespectively in the emitter-base circuits of transistors 12 and 11. Inthese circuits, they supply voltage in addition to the substantiallyconstant drive voltage obtained from transformer 13. The inverse voltagefeedback is obtained directly from transformer 15 with windings 52 and5%, which are respectively connected in the emitter-base circuits oftransistors 11 and 12. The voltage developed by these windings is inopposition to the voltage provided by transformer 13.

By adjustment of resistor 21, the current feedback can be set to a valuewhich will provide relatively constant voltage for normal load currents.In case of an overload, it is necessary to limit the maximum currentwhich can be drawn from the circuit, to protect the circuit 6 elements,particularly transistors 11 and 12. In one embodiment of my invention Iaccomplished this result by causing the core of transformer 14 tosaturate at a predetermined value of load current. For load currents inexcess of this value, the current feedback ratio is substantiallyreduced, thereby limiting the maximum load current which can be drawnfrom the power amplifier.

However, my power amplifying circuit has a current limitingcharacteristic which is not dependent upon the saturation of transformer14. With increasing values of load current, the current gain oftransistors 11 and 12 has a tendency to fall off rather rapidly.Consequently, the output voltage across load 16 falls off at highervalues of load current due to the loss of gain in the transistors. Thischaracteristic is amplified by the fact that the output current falls inproportion to the load voltage, so that the impedance of load 16 can bereduced to very low values or even to a short circuit without damagingthe transistors ll and H. By proportioning the circuit elements so thata relatively large portion of the total driving power is obtained fromthe current feedback circuit, I am able to produce a sharply droopingoutput voltage characteristic which provides excellent protection forthe transistors Ill and lit under overload conditions. At the same time,by deriving a large portion of the driving power from the load circuitinstead of from the driver amplifier, the power requirements placed onthis amplifier become relatively light and readily attained with smallcomponents, which in turn reflect very little power drain on theoscillator 61.

The voltage drops through th circuit resistances in the driving circuitof the power amplifier and in the load circuit of the power amplifierare also determining factors in establishing the amount of currentfeedback which is required. These circuits are, therefore, preferablydesigned with sufilciently high resistance to provide th currentfeedback ratio which corresponds with the required voltage regulationcharacteristic.

By adjustment of resistor 21, the current feedback ratio can be adjustedto obtain the required regulation between no load and rated full load onthe power amplifier, and can also be adjusted to compensate formanufacturing variations in the gain of power transistors 11 and 12.

FIGURE 2 is the schematic diagram of a modified [power amplifierembodying features of my invention.

The circuit of FIGURE 2 can be substituted for a portion of the circuitof FIGURE 1 terminated at terminals 49, 41, 42, 43, and 44. The circuitof FIGURE 2 also comprises a push-pull class B amplifier circuitemploying transistors l1 and 12 and embodying the inverse voltagefeedback and positive current feedback circuits of FIG- URE 1. Thedirect current power supply circuit in FIG- URE 2 differs from that inFIGURE 1 in employing two windings 5'3 and for energizing the outputtransformer 15 in place of the single winding 53 shown in FIGURE 1. Theemitter-collector circuit of transistor 12 is energized from the sourcell) having terminal 44, through winding 55. The emitter-collectorcircuit of transistor 11 is energized from terminal 44 through winding53. The emittor-base circuit of transistor 11 is energized from winding48 through resistor 58, winding 51 and winding 52 as in FIGURE 1. Theemitter-base circuit of transistor 12 is energized from winding 47through resistor 5'7, winding 50, and Winding 54 as in FIGURE 1. Theoperation of the circuit of FIGURE 2 is substantially the same as thatof FIGURE 1, except that the maximum voltage which transistors ll and 12must sustain in FIGURE 2 is equal to slightly more than twice thevoltage of source 10. This circuit is, therefore, adapted for use withhigher voltage transistors, or for operation from a direct currentsource of reduced voltage.

Although the circuits shown employ class B push-pull amplifiers and typePNP transistors in both the driver and power stages, it will be apparentto those skilled in the art that the features of my invention may be em-7. bodied'in other known types of amplifier circuits, such as singleended Class A circuits or with type NPN transistors without departingfrom the scope of my invention.

It is understood that this disclosure has been made only by way ofexample, and that numerous modifications in the choice and arrangementof parts may be resorted to without departing from thespirit and scopeor" the invention as hereinafter claimed.

I claim:

1. In an oscillating circuit energized from a, source of time varyingvoltage, the combination of linear inductance means and capacitancemeans for determining the frequency of oscillation of said circuit, withwave distorting means for introducing high order harmonics into saidcircuit, said wave distorting means comprising a saturable inductanceelement connected in series with said capacitance means and a half-Waverectifier connected substantially in parallel with said saturableinductance element, whereby the effect of said saturable inductanceelement on the frequency of oscillation of said circuit is minimized bysaid half-Wave rectifier.

2. In an oscillating circuit energized from a source of time varyingvoltage, the combination of linear inductance means and firstcapacitance means for determining the frequency of oscillation of saidcircuit, with wave distorting means for introducing high order harmonicsinto said circuit, said wave distorting means comprising a saturable tinductance element connected in series with said first capacitancemeans, a half-wave rectifier, and second capacitance means, saidhalf-wave rectifier being connected substantially in parallel with saidsaturable inductance element, whereby the effect of said saturableinductance element on the frequency of oscillation of said circuit isminimized by said half-wave rectifier, said second capacitance meansbeing vconnected in parallel with said satura'bleinductance element andcomprising with it an oscillating circuit for said high order harmonics.

3. An oscillator'cornprising in combination first and secondtransistors, each having a base, an emitter, and a collector, first andsecond resistors, first, second, and third capacitors, first and seconddirect current circuits, means for energizing said first and seconddirect current circuits 7 in parallel from a source of direct current,said first circuit comprising said first resistor, the. collector ofsaid first transistor and the emitter of said first transistor, saidsecond direct current circuit comprising said second resistor, thecollector of said second transistor and the emitter of said secondtransistor, said first capacitor being connected between the collectorof said first transistor and the base of said second transistor, saidsecond capacitor being connected between the collector of aid secondtransistor and the base of said first transistor, a linear inductanceelement, a saturable inductance element, a half-wave rectifier, saidlinear inductance element being connected between the base of said firsttransistor and the base of said second transistor, said third capacitorbeing connected substantially in parallel with said linear inductanceelement and comprising with it a resonant circuit to determine thefrequency of oscillation of said oscillator, said ond transistors eachhaving a collector, base, and emitter,

first and second'resistors, first, second, third, and fourth capacitors,a linear inductance element, a saturable inductance element, a half-waverectifier, first and second direct current circuits, means forenergizing said first and second direct current circuits substantiallyin parallel from a source of direct current, said first direct currentcircuit comprising said first resistor, the collector of said firsttransistor, and the emitter of said first transistor, said second directcurrent circuit comprising said second resistor, the collector of saidsecond transistor, and the'emitter of said second transistor, said firstcapacitor being connected between the collector of said first transistorand the base of said second transistor, said second capacitorbeingconnected between the collector of said second transistor and the baseof said first transistor, said linear inductance element being connectedbetween the base of said first transistor and the base of said secondtransistor, said third capacitor being'connected substantially inparallel with said linear inductance element and comprising with it aresonant circuit to determine the frequency of oscillation of saidoscillator, said saturable inductance ele- I ment being connected inseries with said third capacitor, said fourth capacitor beingconnectedsubstantially in'parallel with said saturable inductanceelement, said half- Wave rectifier being connected substantially inparallel.

with said saturable inductance element, said halfiwave rectifierby-passing alternate half cycles of current from said third capacitoraround said saturable inductance element to insure freedom ofoscillation of said resonant circuit, said saturable inductance beingsaturated on nonconducting halficycles of said half-wave rectifierandproducing with said fourth capacitor a damped oscillation at afrequency which is a high order harmonic of the freqency of saidoscillator.

References Cited in the tile of this patent UNITED STATES PATENTS2,896,090 Peder July 21, 1959

1. IN AN OSCILLATING CIRCUIT ENERGIZED FROM A SOURCE OF TIME VARYINGVOLTAGE, THE COMBINATION OF LINEAR INDUCTANCE MEANS AND CAPACITANCEMEANS FOR DETERMINING THE FREQUENCY OF OSCILLATION OF SAID CIRCUIT, WITHWAVE DISTORTING MEANS FOR INTRODUCING HIGH ORDER HARMONICS INTO SAIDCIRCUIT, SAID WAVE DISTORTING MEANS COMPRISING A SATURABLE INDUCTANCEELEMENT CONNECTED IN SERIES WITH SAID CAPACITANCE MEANS AND A HALF-WAVERECTIFIER CONNECTED SUBSTANTIALLY IN PARALLEL WITH SAID SATURABLEINDUCTANCE ELEMENT,